The invention relates to a glass composition which is suitable for use primarily in electric light sources.
Some glass parts of electric lamps as stems of incandescent and fluorescent lamps and the exhaust tubes thereof as well as the bulbs of some fluorescent lamps have been made of glasses with relatively high (20 to 29 mass %) lead oxide content for a long time. With these glasses, the high lead oxide content provides for the expected high electric resistance and the acceptable softness needed for good workability.
It is known, however, that lead containing glass exerts harmful effects to the environment both during glass and lamp manufacture and in the form of waste arising at the users. Thus, based on environment protection considerations, one has to make efforts to reduce/eliminate lead content in/from mass products made of glass. A European patent application No. EP 603 933 discloses a lead-free glass composition which is suitable for use in electric lamps, primarily in stem glasses and fluorescent lamp bulb glasses. Although the glass compositions described in it closely approximate the lead-containing ones as to their application parameters (e.g. electric resistance, workability, thermal expansion, etc.), their viscosity values are much higher than those of the lead glass. Due to this, they require higher flame temperatures, i.e. more energy to be supplied during the manufacture of a lamp, which is disadvantageous particularly in the case of high-speed production lines. It is also disadvantageous that these glass compositions contain a substantial amount of barium and strontium oxides (7 to 11 mass % BaO, 1 to 5 mass % SrO). Although BaO is presently qualified as less hazardous to the environment than PbO, it is still strongly poisonous and it should be reduced first of all owing to labor hygiene considerations.
In the glass compositions according to European patent application No. EP 603 933, BaO primarily provides for the high electrical resistance (and serves as a substitute for PbO in this respect), and enables the glass to have good workability. The lower limit of BaO content is a critical value according to this disclosure. Using BaO in amounts below 7 mass % reduces not only the electrical resistance of the glass, but also increases its melting, softening and working temperatures which adversely affects workability. SrO is also needed for ensuring good workability.
We set ourselves the objective of developing a glass composition which is suitable for use primarily in electric lamps and in which glass composition the amounts of BaO and SrO can be reduced and SrO can even be fully eliminated in some cases without the impairment of the electrical resistance and the workability parameters of the glass.